Generally, in small loudspeaker devices, bass reproduction is difficult due to the influence of acoustic stiffness, because the volume of a loudspeaker cabinet is small. In other words, when an electric signal is applied to a loudspeaker, the air within the cabinet is compressed due to vibration of the loudspeaker and the compressed air acts as an air spring and interferes with the movement of the loudspeaker, resulting in a decrease in the sound pressure level, particularly in a bass range. Thus, sufficient bass reproduction cannot be achieved. In order to realize bass reproduction in small loudspeaker devices, there has been proposed a loudspeaker device in which a gas adsorbent material such as activated carbon is disposed in the interior of the cabinet (WO 84/03600, for example).
The loudspeaker device disclosed in WO 84/03600 is composed of: a loudspeaker cabinet; a loudspeaker attached to one face of the cabinet so that a rear portion thereof is in communication with the interior of the cabinet; a gas contained within the cabinet; and a gas adsorbent material such as activated carbon disposed in the cabinet. When an electric signal is applied to the loudspeaker, the gas within the cabinet is rapidly compressed and expanded due to vibration of the loudspeaker. Accordingly, molecules of the gas are adsorbed into and desorbed from the above-described activated carbon. Therefore, pressure fluctuations in the interior of the cabinet are suppressed. As a result, according to the disclosure of WO 84/03600, the sound pressure level in the low frequency range is not decreased, and an effect equal to that in the case where a cabinet having a large capacity is used is attained.
Desirably, the gas adsorbent material, for example, activated carbon, has a low moisture content. The reason for this is that if an activated carbon on which moisture is adsorbed is installed in the cabinet, the activated carbon will show an insufficient ability to adsorb the gas molecules even when the gas within the cabinet is compressed due to vibration of the loudspeaker. Thus, WO 84/03600 above employs a complicated configuration in which a moisture impermeable partition (diaphragm) is located within the cabinet between the loudspeaker and the gas adsorbent material such as activated carbon.
WO 03/013183 discloses the use of an adsorbent material that has been treated to render it at least partially hydrophobic as the gas adsorbent material installed in the cabinet so that the adsorbent material is unlikely to adsorb moisture even in an atmosphere of high humidity. For example, an activated carbon that has been treated by reaction with a silicon-containing compound so as to be hydrophobic is disclosed. GB 2391224A discloses an activated carbon that has been treated so as to be hydrophobic and that can be used as such a gas adsorbent material. Although such a material can be used even in an atmosphere of relatively high humidity, a complicated step of treating the material to render it hydrophobic is required.
WO 03/101147 discloses a loudspeaker assembly in which an activated carbon is installed in a cabinet and the cabinet is purged with a high concentration of dry carbon dioxide gas. The loudspeaker assembly further includes a sensing means for sensing the concentration of carbon dioxide within the cabinet, a means for supplying carbon dioxide, and a means for controlling the supply of carbon dioxide. However, even in this loudspeaker assembly, a complicated means for maintaining the humidity at a low level is required.
Accordingly, there exists a demand for a means for improving bass reproduction in the loudspeaker devices, in particular, for a further improvement of the gas adsorbent material such as activated carbon.